Measurement of the degree of spatial coherence of high-order harmonics using a Fresnel-mirror interferometer -: art. no. 043802

We have studied in detail the spatial coherence of the far field of the 13th harmonic from a Ti:sapphire laser generated in xenon, as a function of the generation parameters. Experimentally, we use Fresnel mirrors to produce two-dimensional interferograms. This technique allows us to probe the coherence at different scales d=1-3 mm between the interfering rays. i.e., throughout the full section of the incident beam. A high uniform degree of mutual coherence gamma(d) , larger than 0.5 in most cases, is measured us a function of the position of the jet relative to the focus, and pressure in the jet. It confirms the high intrinsic spatial coherence already reported for the extreme-ultraviolet harmonics. which is much larger than the one produced from x-ray lasers, Spatial coherence decreases when the laser focus is moved toward the jet, and when the pressure is increased: the onset of ionization, as well as the intensity-dependent phase of the nonlinear polarization, are rapidly varying factors in time and space which degrade the correlation between the fields at two different points. Simulation of the coherence degree emphasizes the role of the intensity-dependent phase in the evolution of the coherence degree.